Poor gas mileage, not sure what else to check

Hello! First of all it’s good to be back! I sold my ‘80 xj650 3 years ago but just bought back the same one! It was abandoned in a storage unit so it was still registered in my name and hadn’t gotten a single mile added to it!

But… after going through the carbs and getting it running, I’ve had a consistent 22-25 mpg. I believe I got this before as well but can’t remember well.

things I’ve done: cleaned the carbs fully, all jets are clear, ultrasonic cleaned, throttle shaft seals replaced when I had the bike 3 yrs ago, diaphragms are solid, slides move free, no vacuum leaks. Checked brakes dragging, no luck. Also checked tire pressure. I believe the carbs weren’t perfectly synched as I have found out after tearing them apart but they were close... valves are done but my #2 exhaust is a little loose but not something that would affect mpg this much. Plugs are pretty decent ranging from a light paper bag brown to a darker grey ish color nothing hinting super rich. All my cylinder psi is around 125 hot at 6,000 feet elevation so that’s still in spec, and not enough to throw the mpg. Petcock is good too, rebuilt that 3 years ago. Tank is off and it’s not leaking a drop.

I haven’t cleaned the k&n air filter. Just thought of that. New intake boots are on the way just in case, as well as fork seals and such. Needles and emulsion tubes look ok but I’m not sure what to check if they are out of spec. Jets are a bit damaged and scored from probably me 3 yrs ago trying to pull them off. I found out today that #4 mixture screw is missing the tip and #2 o ring was broken as well. I can see that making a tiny difference but not 25mpg diff… I can’t find just the jets for sale on eBay anywhere, or the emulsion/needles too… I’m trying to get the thing going before Monday. Also figure this might be added, im getting a slight stutter at 3500rpm. I wish I could lower the needle a notch and play around but I can’t because they are not adjustable >:/

I’m really wanting this bike to get good mpg’s cuz I just finished building a 2332 vw engine and that now gets the worst and it’s my second daily driver. Hoping my this bike can ~5x that economy… also I found a carb set locally to me. Not sure if it’s any better but might be worth a shot. It’s a low mileage bike so I assume parts have less wear, might be able to swap all the parts to my carbs and see what happens but idk if it’s worth the asking price :/

 

Why is the left plug different?

 

Looks like its running rich. Higher elevation, you need more air. Do you know what jets are installed?
people picking clean the jets and being too agressive can open up the jets.
Also, you need to know how its wet leveled on the carb floats, if too high, that richens things up.

 

Just noticed that too. I’ve got no clue. Will be changing them all after going through the bike.

 

Stock jets, 40 and 110. I believe the other 4 or so are stock too. The bike only needs a bit of choke to start and idle so I know it’s a tad rich but it won’t idle without the choke when cold either. Is xj4ever the only place to source hitachi jets?

 

Are they all the same heat range?

 

I believe so. I’ll have to double check the numbers. At least they are all non-resistor plugs haha

 

yea, that left plug is sunken in or something.
xj4ever is the only place I know of to get jets that are truely trust worthy unless you want to drill your own.

I think you should check your wet level cause you could possibly drop your float fill level 2mm across all 4 and get more air. (may be a bad idea but if you have the time to try it, may work for you).

 

Yes it is sunk in something clearly wrong.

 

I'm curious to know how much float level affects rich/lean. With all the discussions on jetting changes, I never see anybody suggest this. It seems like it would be one of those old school tuning tricks to simulate jet changes, or fine tune between jet sizes.

But then the difference between atmospheric and vacuum is something like 14,000 mm of gasoline column, so 1 or 2 mm feels like a pretty small impact on differential when a carb might use 75% of that range between zero and full vacuum.

Not to suggest that fuel level is unimportant, just wondering where it is on the magnitude level as compared to jetting, air leaks, pilot adjustment, butterfly synch, air filter restriction, valve leakage, etc.

 

I’ve plated with it quite a bit on mopeds and I’ve gotten them to run quite nicely just by adjusting the float. That is only one float though. I’ve wet set all of these as well. I seriously don’t know what to check next!

 

My Seca 750 the PO had installed a K&N filter, I put back in a paper element and it ran much better and mileage went up . Still have the K&N in a box , after doing a full rebuild and synch for giggles put this filter in , ran poorly hence while it is in a box.

 

Was that a cloth filter or foam?

 

Just to further this conversation:

Is paper OEM style the only recommended filter type?

I have a foam “Uni” filter and a K&N (wet, reusable). I don’t have either of my bikes dialed in quite yet and I’m thinking of buying a paper filter

 

I have a foam Uni filter on my XS650 (twin). Per the suggestion of the experts, I bumped up the pilot jet 2 sizes , up the main jet 2 sizes and lowered (leaned) the needle one slot (the XS has slots to move the needle unlike the XJ).
The XS I have does not have a screw drain so when I first set up the carbs, I could not wet set. Then purchased a special drain plug w/ hose to check wet level and I was about 2mm too low and that carb was running lean w/ stock filter. Moving it up it ran noticably better.

On an XJ that was running lean, I bumped the pilot from 40 to 42 and put one washer on the needle. I always wet set my XJ carbs to as close as 3mm as I can get. I also make sure both floats dry measure the same height cause they can be bent and when you measure just one side, the other side skews the results.

I only mention to you about adjusting the floats cause its free ($ not time) if you don't want to re jet and spend the $.

 

I asked because the X I bought has a cloth /oil filter and I bought a foam /oil filter to replace it because I read somewhere it is close to using a paper filter. I haven't installed the foam filter yet.

 

I just ordered a paper filter for it just in case. I’ve found that one mixture screw has a broken tip but nothing left in the carbs which is nice. I’ll have to get a new one of those. Plus one of the rubber o rings on those screws was cracked. Floats have been wet set several times. Not sure if they are wonky but thats a good point! I’ll check those next. I found a set of carbs near me I might buy too. Can’t hurt to have another set for spare parts…

 

If your filter is foam/oil filter then your bike will run rich which is what happened to me. It's a pretty common thing when using those foam filters. You can still use them but would have to re-jet. Also, I had extremely poor gas mileage when I used aftermarket emulsion tubes. They looked EXACTLY like the originals but my bike was never the same. I road like that for 3 years thinking the issue was something else. It was just by chance that I came across an article explaining how crucial it is to have the OEM emulsion tubes in your carbs. I luckily still had my originals, popped them in, issue resolved.

 

I’m buying another carb set tomorrow that is stock. I’ll be swapping emulsion tubes and needles since it’s a low mileage bike, but I believe mine are stock… gonna swap the jets too because mine are a bit scratched and marred from me learning how to work on carbs 3 years ago haha. So replacing those will hopefully help! I ordered a paper filter too so from what I’ve heard I’m hoping the gas mileage increases a fair amount!

 

So I do have an update. I bought the set of carbs and they were pure virgin carbs! The idle mixture screws were still covered. 2 things I found out. The emulsion tubes and needles are original and comparing mine and those closely, I’ve found mine aren’t! I think mine were unlabeled and oddly shaped, new needles are like y10 or something, so I swapped those out. Jets were also in a lot better condition so I swapped those. Then I found out the idle mixture screws seemed newer so I swapped those out too. If I hadn’t done a full clean on my carbs and redone the throttle shaft seals, I probably woulda used this set of carbs!!! Trying out the new filter tonight and will probably wet-set the carbs… hopefully I’m narrowing down the issues!

 

Ok another update. I got all the internals swapped and finally got it running decently. My butterfly’s could be set better but it’s pretty good. Now my question is… i put the carbs under a small amount of pressure sealing off the inlet and exit and am leaking air through the chokes. I assume the seal isn’t supposed to fully seal this. Valve grinding compound on the seat? Would that be the way to go? I got them to seal fine for now until I get the fork seals fixed, but looking for a near future good fix.